<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    .Grid {
      display: flex;
    }

    .Grid-cell {
      flex: 1;
      background-color: #ddd;
      height: 60px;
      width: 500px;
      margin-right: 10px;
      text-align: center;
      line-height: 60px;
    }
    .Grid {
      display: flex;
    }

    .Grid-cell {
      /*flex: 1 1 auto;*/
      flex: auto
    }
    .first {
      width: 200px;
    }
    .two {
      width: 300px;
    }
    .Grid-cell1 {
      background-color: #ddd;
      height: 60px;

      margin-right: 10px;
      /*flex: 0 0 auto;*/
      margin-top: 10px;
    }
    .left {
      width: 33%;
    }
    .right {
      width: 67%;
    }
  </style>
</head>
<body>
<div class="Grid">
  <div class="Grid-cell first"></div>
  <div class="Grid-cell two"></div>
  <div class="Grid-cell three"></div>
</div>
<hr>
<div class="Grid">
  <div class="Grid-cell">1/2</div>
  <div class="Grid-cell">1/2</div>
</div>
<div class="Grid">
  <div class="Grid-cell1 left">1/3</div>
  <div class="Grid-cell1 right">2/3</div>
</div>
</body>
</html>
